Discussions
Issues of estimating canopy rainfall retention have been investigated by
various authors. However, most research is devoted to precipitation
losses for long periods, but few publications focus on precipitation
interception of a single rainfall event.
The first known studies on precipitation losses of a single rainfall
event by forest canopies were carried out in the USA (Horton, 1919) and
Switzerland (Buhler, 1891) in the late 19th–early 20th century. The
studies relied on parallel observations of the field precipitation
gauges installed both under the middle part of the canopies of the
individual deciduous species and in open areas.
In 1917–1919, the Horton’s laboratory determined the values of maximum
water retention by the canopies of 12 deciduous tree species (Norway
maple (Acer platanoides L.), elm (Ulmus spp.), white willow (Salix alba
L.), European ash (Fraxinus excelsior L.), beech (Fagus spp.), American
linden (Tilia Americana L.), English oak (Quercus robur L.), Eastern
hemlock (Tsuga canadensis (L.) Carrière), Scots pine (Pinus sylvestris
L.), hickory (Carya spp.), European horse-chestnut (Aesculus
hippocastanum L.), apple tree (Malus spp.), varying from 0.5 to 2.9 mm
per canopy projection area. It was found that during the first minutes
of the rain, from 70% to 100% of the precipitation is kept by the
canopies (i.e., the proportion of precipitation leaked from the start of
the rain to the moment of full water saturation of the tree canopies
makes 25%–30%).
The general Horton model determines the amount of interception as the
difference between the amount of precipitation flowing under the forest
canopy and the amount of moisture retained by canopies and lost due to
evaporation from the leaf surface.
The most important regularities of rainfall redistribution under the
forest canopy were revealed by the beginning of the 20th century in the
studies of Ebermeyer, Goppe, N.S. Nesterov, S.D. Okhlyabinin and others.
The precipitation retention by vegetation cover and its subsequent
evaporation are directly related to the size and properties of the
wetted surface. The studies of different authors show that maximum
canopy water retention differ significantly. According to Bele J. (Bele,
1975, 1980), spruce stands can intercept up to 3.6 mm of a single
rainfall event. V.V. Rakhmanov (V.V. Rakhmanov, 1981) determined that
the water retention capacity for coniferous stands (spruce, fir) is
within 2-4 mm (in some cases - up to 6-8 mm).
The total interception is dependent on the age, composition, taxational
stand volume determining the area of the leaf surface and the amount of
precipitation. Light rains (0.5-1.5 mm) are completely intercepted by
tree stand canopies; the maximum moisture retention capacity of
high-density coniferous trees is 10-12 mm (Onuchin A.A., 2003).
Point estimates of the layer of single rainfall interception received by
the authors slightly differ from the results obtained earlier. The
difference stem from the fact that for understanding of hydrological
processes the authors carried out artificial sprinkling of tree species
and the volume of moisture retention was determined by weight
experiment. In other studies, these values were determined by making
observations of net and gross rainfalls which have a number of
uncertainties related to the choice of precipitation measurement points
under the canopies of individual trees.
In recent years, the research results on determining moisture retention
values for such species as Ulmus processra, Platanus×acerifolia and
Corymbia maculata per leaf surface area have been covered in works by
Mariana D. Baptista et al. (2018). It was found that the interception
value is determined by the leaf surface area and precipitation
intensity. Thus, methods similar to those of the authors of this study
were used to determine the leaf surface area. At the same time,
interception storage capacity mapping within the whole river watershed
has never been carried out.
In order to estimate the depth of precipitation distributed in space h
(mm) and irrecoverably retained by tree canopies, the following ratio
proceeding from the dependencies presented in Fig. 3 is used:
,
where k is the stand density, γ is specific water retention (water mass
(kg) per 1 m2 of leaf surface area LA). Specific water
retention value γ for forest forming species is calculated as weighted
average proceeding from the share of species in the watershed area,
based on empirical dependencies:
γ=167 - for coniferous stands, γ=92.7 - for deciduous stands, γ=108 -
for mixed deciduous stands, γ=151 - for mixed coniferous stands.
In its turn, the LAI value was estimated for the main stand types by the
formulas derived from empirical dependencies proposed by Utkin and
calculated as weighted average for forest forming species of the model
watershed (Table 4).
Based on the above-mentioned ratios, calculations and experimental
works, the values of irrecoverable rainfall retention by canopies were
calculated and mapped. All the previously obtained data related to the
studied area were brought into a single thematic layer by methods of
GIS-technologies and spatial analysis operations. After that, based on
the data from the attribute table of a layer, the data were recalculated
on the above-mentioned formula with the use of Field Calculator tools in
ArcGIS 10.4. The obtained data have been classified by the method of
defined intervals and are shown on the map (Fig. 8).
As can be seen from the results of the calculations, common larch has
the highest water-retention capacity (up to 147 liters of water for a
given bonitet and age), and aspen has the lowest - (13.3 liters of
water). The layer of the initial rainfall interception reaches 5.9 mm
per 1 ha of forest-covered area. With the change in age and bonitet
class of natural tree stands, the precipitation interception layer can
reach per 1 ha can be as high as 5-8 mm or even more. Due to the uneven
distribution of forest-forming species in the watershed area, the amount
of precipitation actually flowing under the canopy differs significantly
from the amount of precipitation falling on the canopy. It was the first
case in the world scientific practice when mapping (spatial distribution
of precipitation losses by canopies) was carried out.